In accordance with demand for higher performance and/or higher quality of a liquid crystal display device, various requirements have been made on polarizing plate protective film.
As ordinary polarizing plate protective film for a liquid crystal display device, primarily utilized is film comprised of cellulose ester as a raw material. Cellulose ester film production generally employs a solution casting method in view of enhanced flatness, and also since said method tends to decrease refractive index in the thickness direction compared to the refractive index in the plane of the film.
In Patent Document 1, cellulose ester film, the optical anisotropy of which has been decreased by addition of an ethylenical polymer, is described, and film can be prepared in which particularly the retardation value in the thickness direction Rt can be decreased.
Further, an in-plain switching mode type (hereinafter, referred to as an IPS type) liquid crystal display device, to improve display ability (viewing angle, color, and gray scale, etc.) of a liquid crystal display device, it is more preferable to utilize a polarizing plate employing cellulose ester film exhibiting a low retardation value compared to a polarizing plate employing conventional cellulose ester, and film described in Patent Document 1 described above, among others, exhibits excellent optical characteristics.
However, when a polarizing plate incorporating a cellulose ester film, the optical anisotropy of which has been decreased, is utilized in a liquid crystal display device, it has been proven that variation of visual recognition such as contrast decreases and color shift due to excessive heat from a backlight, in the case of being used under an environment of such as high temperature and high humidity for a long duration, which required rushed improvement.    [Patent Document 1] Unexamined Japanese Patent Application Publication No. (herein after, referred to as JP-A) 2003-12859